Managing granularity of a metadata structure for a storage system

ABSTRACT

Techniques are provided for managing metadata of a storage system. A storage control system manages a storage metadata structure which comprises metadata items for accessing data items in a storage system comprising a persistent write cache and a primary storage. The storage metadata structure comprises a first metadata structure that is configured to organize metadata items received from the write cache at a first granularity, and a second metadata structure that is configured to organize metadata items received from the first metadata structure at a second granularity, wherein the second granularity is greater than the first granularity. The storage control system utilizes the storage metadata structure to access data items in the persistent write cache and the primary storage.

TECHNICAL FIELD

This disclosure relates generally to data storage management techniques and, more particularly, to techniques for managing metadata in a data storage system.

BACKGROUND

Storage systems utilize some form of internal layout for a physical data layer, and employ a mapping mechanism from a logical layer (as understood by user volumes or files) to a physical layer that is used to store data. A storage controller may arrange data in the physical layer using various methods such as, e.g., packing data to conserve capacity, implementing a log-structured array, storage tiering, etc. In addition, storage systems require various types of metadata to support core storage functionality. Such metadata includes, e.g., metadata for mapping logical locations (offset in a file or volume) to a physical location (to track the physical location of stored data items), invalidation and garbage collection related metadata, metadata for accounting, deduplication referencing, snapshot generation, and tracking relationships, and resiliency related metadata (e.g., RAID), etc. The metadata must be resilient to failures such as a device or a node going offline, because without the metadata, the stored data become inaccessible and key functionality breaks. In addition, the process of persisting metadata should not add a significant amount of bandwidth or IOPS (input/output operations per second) which degrades system performance and affects the endurance of storage devices used to store the metadata. In this regard, efficient metadata management is critical to overall performance of a data storage system.

SUMMARY

Exemplary embodiments of the disclosure include techniques for managing metadata. For example, an exemplary embodiment includes a method that is performed by a storage control system. The storage control system manages a storage metadata structure which comprises metadata items for accessing data items in a storage system comprising a persistent write cache and primary storage. The storage metadata structure comprises a first metadata structure that is configured to organize metadata items received from the write cache at a first granularity, and a second metadata structure that is configured to organize metadata items received from the first metadata structure at a second granularity, wherein the second granularity is greater than the first granularity. The storage control system utilizes the storage metadata structure to access data items in one of the persistent write cache and the primary storage.

In another exemplary embodiment, the first granularity corresponds to a first allocation unit size of the persistent write cache, wherein each metadata item with the first granularity references a data item having the first allocation unit size. The second granularity corresponds to a second allocation unit size of the primary storage, which is greater than the first allocation unit size of the persistent write cache, wherein each metadata item with the second granularity references a data item with the second allocation unit size.

In another exemplary embodiment, the first metadata structure comprises a log-structured merge tree data structure, and the second metadata structure comprises a B+ tree data structure.

Other embodiments of the disclosure include, without limitation, systems and articles of manufacture comprising processor-readable storage media, which are configured for managing metadata of a storage system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a network computing system comprising a data storage system which implements a metadata management system, according to an exemplary embodiment of the disclosure.

FIG. 2 schematically illustrates a storage node which comprises a metadata management system, according to an exemplary embodiment of the disclosure.

FIG. 3 schematically illustrates a process for utilizing a high-performance data structure for managing metadata in a storage system, according to an exemplary embodiment of the disclosure.

FIG. 4 schematically illustrates an exemplary log-structured merge tree data structure which is utilized to implement a high-performance data structure for managing metadata in a storage system, according to an exemplary embodiment of the disclosure.

FIG. 5 schematically illustrates an exemplary B+ tree data structure which is utilized to implement a high-performance data structure for managing metadata in a storage system, according to an exemplary embodiment of the disclosure.

FIG. 6 schematically illustrates an exemplary log-structured merge tree data structure which is configured to manage metadata items at sub-chunk granularity, according to an exemplary embodiment of the disclosure.

FIG. 7 illustrates a flow diagram of a method for managing metadata using a metadata structure that is configured to manage metadata items at chunk and sub-chunk granularities, according to an exemplary embodiment of the disclosure.

FIG. 8 schematically illustrates a framework of a server node for hosting a storage node which comprises a metadata management system, according to an exemplary embodiment of the disclosure.

DETAILED DESCRIPTION

Exemplary embodiments of the disclosure will now be discussed in further detail with regard to systems and methods for managing metadata in a storage system. For purposes of illustration, exemplary embodiments will be described herein with reference to exemplary network computing environments, such as distributed storage environments, which implement data processing systems and associated computers, servers, storage devices and other processing devices. It is to be appreciated, however, that these and other embodiments are not restricted to the particular illustrative system and device configurations shown. Accordingly, the term “network computing environment” as used herein is intended to be broadly construed, so as to encompass, for example, processing systems comprising cloud computing and storage systems, as well as other types of processing systems comprising various combinations of physical and virtual processing resources. A network computing environment may therefore comprise, for example, at least one data center or other cloud-based systems that include one or more cloud systems that host multiple tenants which share cloud resources. Numerous different types of enterprise computing and storage systems are also encompassed by the term “network computing environment” as that term is broadly used herein

FIG. 1 schematically illustrates a network computing system comprising a data storage system which implements a metadata management system, according to an exemplary embodiment of the disclosure. The network computing system 100 comprises one or more host systems 110-1, 110-2, . . . 110-H (collectively, host systems 110), one or more management nodes 115, a communications network 120, and a data storage system 130. The data storage system 130 comprises one or more storage nodes 140-1, 140-2, . . . , 140-N (collectively, storage nodes 140). As shown in FIG. 1 , the storage node 140-1 comprises a storage control system 150, a storage array 160 (e.g., primary storage) comprising a plurality of storage devices 162-1, . . . , 162-D (collectively, storage devices 162), and primary memory 170 (alternatively, system memory 170). The primary memory 170 comprises volatile random-access memory (RAM) and non-volatile RAM (NVRAM). The storage control system 150 comprises a storage data server 152, and a metadata management system 154. The primary memory 170 is configured to implement a write cache 180 and a primary metadata structure 190 (e.g., storage metadata structure). The metadata management system 154 implements methods that are configured to provision and manage the primary metadata structure 190. As explained in further detail below, the metadata management system 154 is configured to utilize the write cache 180 and the primary metadata structure 190 to manage metadata on the storage node 140. In some embodiments, the other storage nodes 140-2 . . . 140-N have the same or similar configuration as the storage node 140-1 shown in FIG. 1 .

In general, the management nodes 115 implement application programming interfaces (APIs) to enable manual, automated, and/or semi-automated configuration, management, provisioning, and monitoring of the data storage system 130 and the associated storage nodes 140. In some embodiments, the management nodes 115 comprise stand-alone dedicated management server nodes, which may comprise physical and/or virtual server nodes.

The host systems 110 comprise physical server nodes and/or virtual server nodes which host and execute applications that are configured to process data and execute tasks/workloads and perform computational work, either individually, or in a distributed manner, to thereby provide compute services to one or more users (the term “user” herein is intended to be broadly construed so as to encompass numerous arrangements of human, hardware, software or firmware entities, as well as combinations of such entities). In some embodiments, the host systems 110 comprise application servers, database servers, etc. The host systems 110 can include virtual nodes such as virtual machines and container systems. In some embodiments, the host systems 110 comprise a cluster of computing nodes of an enterprise computing system, a cloud-based computing system, or other types of computing systems or information processing systems comprising multiple computing nodes associated with respective users. The host systems 110 issue data access requests to the data storage system 130, wherein the data access requests include (i) write requests to store data in the storage arrays 160 of the storage nodes 140 and (ii) read requests to access data that is stored in the storage arrays 160 of the storage nodes 140.

The communications network 120 is configured to enable communication between the host systems 110 and the storage nodes 140, and between the management nodes 115, the host systems 110, and the storage nodes 140, as well as to enable peer-to-peer communication between the storage nodes 140 of the data storage system 130. In this regard, while the communications network 120 is generically depicted in FIG. 1 , it is to be understood that the communications network 120 may comprise any known communications network such as, a global computer network (e.g., the Internet), a wide area network (WAN), a local area network (LAN), an intranet, a satellite network, a telephone or cable network, a cellular network, a wireless network such as Wi-Fi or WiMAX, a storage fabric (e.g., IP-based or Fiber Channel storage fabric), or various portions or combinations of these and other types of networks. In this regard, the term “network” as used herein is therefore intended to be broadly construed so as to encompass a wide variety of different network arrangements, including combinations of multiple networks possibly of different types, which enable communication using, e.g., Transfer Control Protocol/Internet Protocol (TCP/IP) or other communication protocols such as Fibre Channel (FC), FC over Ethernet (FCoE), Internet Small Computer System Interface (iSCSI), Peripheral Component Interconnect express (PCIe), InfiniBand, Gigabit Ethernet, etc., to implement I/O channels and support storage network connectivity. Numerous alternative networking arrangements are possible in a given embodiment, as will be appreciated by those skilled in the art.

The data storage system 130 may comprise any type of data storage system, or combination of data storage systems, including, but not limited to, a storage area network (SAN) system, a network-attached storage (NAS) system, a direct-attached storage (DAS) system, dynamic scale-out data storage systems, or other types of distributed data storage systems comprising software-defined storage, clustered or distributed virtual and/or physical infrastructure. The term “data storage system” as used herein should be broadly construed and not viewed as being limited to storage systems of any particular type or types. It is to be noted that each storage node 140 and its associated storage array 160 is an example of what is more generally referred to herein as a “storage system” or a “storage array.” The storage nodes 140 can be physical nodes, virtual nodes, and a combination of physical and virtual nodes.

In some embodiments, the storage nodes 140 comprise storage server nodes (e.g., server node 800, shown in FIG. 8 ) having processor and system memory, and possibly implementing virtual machines and/or containers, although numerous other configurations are possible. In some embodiments, one or more of the storage nodes 140 can additionally implement functionality of a compute node, and vice-versa, wherein a compute node is configured to process data and execute tasks/workloads and perform computational work, either individually, or in a distributed manner, to thereby provide compute services such as execution of one or more applications on behalf of one or more users. In this regard, the term “storage node” as used herein is therefore intended to be broadly construed, and a storage system in some embodiments can be implemented using a combination of storage nodes and compute nodes. In some embodiments, each storage node 140 comprises a server node that is implemented on, e.g., a physical server machine or storage appliance comprising hardware processors, system memory, and other hardware resources that execute software and firmware to implement the functionalities and data management services of the storage node 140 and the storage control system 150, as discussed herein. More specifically, in some embodiments, each storage node 140 comprises a plurality of storage control processors which execute a lightweight operating system (e.g., a customized lightweight Linux kernel) and functional software (e.g., software-defined storage software) to implement various functions of the storage node 140 and the storage control system 150, wherein such functions include, but are not limited to, (i) managing and executing data access requests issued by the host systems 110, (ii) performing various data management and storage services, and (iii) controlling network communication and connectivity with the host systems 110 and between the storage nodes 140 within the data storage system 130, etc.

In a distributed storage environment, the storage control systems 150 of the storage nodes 140 are configured to communicate in a cooperative manner to perform functions such as e.g., processing data access requests received from the host systems 110, aggregating/pooling the storage capacity of the storage arrays 160 of the storage nodes 140, performing functions such as inline data compression/decompression, data deduplication, thin provisioning, and data protection functions such as data replication, snapshot, and data protection and resiliency schemes based on data striping and/or parity (e.g., erasure coding, RAID, etc.), and other types of data management functions, depending on the system configuration.

The storage devices 162 comprise one or more of various types of storage devices such as hard-disk drives (HDDs), solid-state drives (SSDs), Flash memory cards, or other types of non-volatile memory (NVM) devices including, but not limited to, non-volatile random-access memory (NVRAM), phase-change RAM (PC-RAM), magnetic RAM (MRAM), etc. In some embodiments, the storage devices 162 comprise flash memory devices such as NAND flash memory, NOR flash memory, etc. The NAND flash memory can include single-level cell (SLC) devices, multi-level cell (MLC) devices, triple-level cell (TLC) devices, or quad-level cell (QLC) devices. These and various combinations of multiple different types of storage devices may be implemented in the data storage system 130. In this regard, the term “storage device” as used herein should be broadly construed to encompass all types of persistent storage media including hybrid drives.

In some embodiments, the storage array 160 of a given storage node 140 comprises DAS resources (internal and/or external), wherein the storage control system 150 of the given storage node 140 is configured to directly access the storage array 160 of the given storage node 140. In some embodiments, the data storage system 130 comprises a disaggregated data storage system in which storage data processing is separate from data storage. More specifically, in an exemplary embodiment of a disaggregated storage system, the storage control systems 150 comprise storage control nodes, and the storage arrays 160 comprise storage nodes, which are separate from the storage control nodes. In such a configuration, the storage control nodes (e.g., storage control systems 150) are configured to handle the processing of data associated with data access requests (i.e., input/output (I/O) read and write requests), and the storage nodes (storage array 160) are configured to handle writing/reading data to/from respective storage devices 162. In a disaggregated architecture, each storage control node (e.g., each storage control system 150) would be configured to directly access data stored in each storage node (e.g., each storage array 160) in the data storage system 130. The disaggregated storage system architecture essentially separates the storage control compute layers (e.g., storage control systems 150) from the data storage layers (e.g., storage arrays 160).

In a disaggregated data storage system, each storage array 160 is implemented as, e.g., an external DAS device, wherein each storage control system 150 of each storage node 140-1, 140-2, . . . , 140-N is connected to each storage array 160 using any suitable interface protocol such as Small Computer Systems Interface (SCSI), Fibre Channel (FC), etc. In other embodiments, the storage control systems 150 of the storage nodes 140-1, 140-2, . . . , 140-N can be network-connected to each of the storage arrays 160 (via a high-performance network fabric) using any suitable network configuration and network interface protocol such as Ethernet, FC, Internet Small Computer Systems Interface (iSCSI), InfiniBand, etc. For example, in some embodiments, the storage nodes 140 and the storage arrays 160 are interconnected in a full-mesh network, wherein back-end interconnectivity between the storage nodes 140 and the storage arrays 160 is achieved using, e.g., a redundant high-speed storage fabric, wherein the storage control systems 150 can utilize remote procedure calls (RPC) for control messages and remote direct memory access (RDMA) for moving data blocks.

In some embodiments, the storage data servers 152 of the storage nodes 140 are configured to consolidate the capacity of the storage arrays 160 (e.g., HDDs, SSDs, PCIe or NVMe flash cards, etc.) of the storage nodes 140 into storage pools from which logical volumes are allocated, wherein the logical volumes (e.g., a block unit of storage management) are identified by, e.g., logical unit numbers (LUNs). More specifically, the storage data servers 152 of the storage nodes 140 are configured to create and manage storage pools (e.g., virtual pools of block storage) by aggregating storage capacity of the storage arrays 160 of the storage nodes 140 and dividing a given storage pool into one or more volumes, wherein the volumes are exposed to the host systems 110 as block devices. For example, a virtual block device can correspond to a volume of a storage pool. Each virtual block device comprises any number of actual physical storage devices, wherein each block device is preferably homogenous in terms of the type of storage devices that make up the block device (e.g., a block device can include only HDD devices or SSD devices, etc.).

In some embodiments, each host system 110 comprises a storage data client (SDC) which executes on the host system and which consumes the block storage exposed by the storage data servers 152. In particular, an SDC comprises a lightweight block device driver that is deployed on a given host system 110 to expose shared block volumes to the given host system 110. The SDC exposes the storage volumes as block devices to each application (e.g., virtual machine, container, etc.) that execute on the same server (e.g., host system 110) on which the SDC is installed. The SDC of a given host system 110 exposes block devices representing the virtual storage volumes that are currently mapped to the given host system 110. The SDC for a given host system 110 serves as a block driver for the host system 110, wherein the SDC intercepts I/O requests, and utilizes the intercepted I/O request to access the block storage that is managed by the storage data servers 152. The SDC provides the operating system or hypervisor (which runs the SDC) access to the logical block devices (e.g., volumes). Each SDC has knowledge of which storage data servers 152 hold (e.g., own) their block data, so multipathing can be accomplished natively through the SDCs.

As noted above, the management nodes 115 in FIG. 1 implement a management layer which manages and configures the network computing system 100. In some embodiments, the management nodes 115 comprise a tightly-coupled cluster of manager nodes that are configured to supervise the operations of the storage cluster and manage storage cluster configurations. For example, management nodes 115 include metadata manager (MDM) modules that operate outside of the data path and provide the relevant information to the SDCs and the storage data servers 152 to allow such components to control data path operations. The MDM modules are configured to manage the mapping of SDCs to the storage data servers 152 of the storage nodes 140. The MDM modules manage various types of metadata that are required to perform various management operations in the storage environment such as, e.g., managing configuration changes, managing the SDCs and storage data servers 152, maintaining and updating device mappings, maintaining management metadata for controlling data protection operations such as snapshots, replication, RAID configurations, etc., managing system capacity including device allocations and/or release of capacity, performing operations for recovery from errors and failures, and performing system rebuild tasks including rebalancing, etc.

The metadata management system 154 is configured to utilize the write cache 180 and the primary metadata structure 190 to manage metadata of the storage system. The write cache 180 and the primary metadata structure 190 are maintained in a persistent storage/memory resource. In the context of a software-defined storage system, the storage control system 150 is essentially a hardware independent storage control system which is configured to abstract storage and memory resources from the underlying hardware platform for greater flexibility, efficiency and faster scalability. In this regard, the storage control system 150 will have no control over the types of storage and memory hardware resources that will be utilized during run-time. In this regard, in some embodiments, the write cache 180 and the primary metadata structure 190 are implemented in primary memory 170. In other embodiments, the write cache 180 and/or the primary metadata structure 190 can be implemented in primary storage (e.g., the storage array 160).

As noted above, the primary memory 170 comprises volatile RAM such as dynamic RAM (DRAM), synchronous DRAM (SDRAM), etc. In some embodiments, the primary memory 170 comprises non-volatile memory which is configured as RAM. For example, in some embodiments, the primary memory 170 comprises a storage class memory (SCM) tier which extends the RAM that is available to the operating system of the storage node 140. The SCM tier can be implemented with various types of non-volatile memory media hardware such as persistent memory (PMEM) modules, solid-state drive (SSD) devices, nonvolatile dual in-line memory modules (NVDIMMs), and other types of persistent memory modules with a DRAM form factor, etc. In addition, the persistent memory may be implemented using a vaulting RAM system which comprises a battery-backed RAM in which data is stored to vault devices upon device or power failure. In general, the non-volatile memory devices can be accessed over a memory bus (implemented via, e.g., Peripheral Component Interconnect Express) using a suitable interface such as non-volatile memory express (NVMe).

The metadata management system 154 is configured to implement a high-performance metadata storage and processing system through use of the primary metadata structure 190. For example, the metadata is maintained and managed in such a manner that the metadata is at least as resilient as the associated data. In addition, the metadata is maintained and managed in a manner that the metadata is swappable between RAM and persistent storage. Due to the size of the metadata, the metadata may not fit within RAM in many cases. As such, the metadata management system 154 allows relevant metadata is be maintained in RAM, and then allows swapping in of metadata when the workload changes. Further, the metadata is generated and managed in a way that minimizes write overhead. For example, the persisting of the metadata does not add a significant amount of bandwidth or IOPS. In addition, the metadata management system 154 supports both random and sequential access to the metadata.

FIG. 2 schematically illustrates a storage node 200 which comprises a metadata management system, according to an exemplary embodiment of the disclosure. In some embodiments, FIG. 2 schematically illustrates an exemplary architecture of the storage nodes 140 of the data storage system 130 of FIG. 1 . As shown in FIG. 2 , the storage node 200 comprises a storage control system 210 which implements a storage data server 220, a data management services module 225, a write cache management system 230, and a metadata management system 240. The storage data server 220 comprises a storage virtualization management module 222. The write cache management system 230 comprises various functional modules including, but not limited to, a write cache destage control module 232. The metadata management system 240 comprises various functional modules including, but not limited to, a first metadata structure management module 242 (alternatively referred to herein as an “updates data structure” management module 242), and a second metadata structure management module 244 (alternatively referred to herein as a “core data structure” management module 244).

The storage node 200 further comprises an array of storage devices 250 and primary memory 260. The storage devices 250 comprise primary storage resources, wherein at least some capacity of the storage devices 250 is partitioned into one or more storage volumes 252. In the exemplary embodiment of FIG. 2 , the primary memory 260 comprises a write cache 262 which is provisioned and managed by the write cache management system 230. In some embodiments, the write cache 262 resides in a region of non-volatile RAM (e.g., PMEM memory, SSD memory, etc.), which is allocated for the write cache 262. In other embodiments, the write cache 262 resides in an allocated region of the storage space of the array of storage devices 250.

As further shown in FIG. 2 , the primary memory 260 comprises a primary metadata structure 270 (e.g., storage metadata structure). The primary metadata structure 270 comprises a first data structure 280 (referred to as the updates data structure 280), and a second data structure 290 (referred to as the core data structure 290). The updates data structure 280 is provisioned and managed by the updates data structure management module 242, and the core data structure 290 is provisioned and managed by the core data structure management module 244. In some embodiments, the updates data structure 280 comprises a log-structured merge (LSM) tree data structure 282, and the core data structure 290 comprise a B+ tree data structure 292, the structures and functions of which will be discussed in further detail below.

The storage data server 220 implements functions as discussed above such as processing I/O write and read requests received from host systems to write/read data to/from the storage devices 250. The storage virtualization management module 222 implements any suitable logical volume management (LVM) system which is configured to create and manage the storage volumes 252 by aggregating the capacity of the storage devices 250 into one or more virtual storage pools that are thin-provisioned for maximum capacity, and logically dividing each storage pool into one or more storage volumes that are exposed as block devices (e.g., LUNs) to the applications or host systems 110 (FIG. 1 ) which consume the data. The data management services module 225 implements one or more types of data management services including, but not limited to, inline data compression/decompression, thin provisioning, and data protection functions such as data replication, data backup, data snapshot, and data protection and resiliency schemes based on data striping and/or parity (e.g., erasure coding, RAID, etc.), and other types of data management functions, depending on the system configuration. In embodiments where the storage data server 220 abstracts the physical media (e.g., storage devices 250) and presents logical (virtualized) addresses to users in the form of LUNs, the storage data server 220 generates metadata to provide mapping between logical addresses and physical addresses. In addition, the storage control system 210 generates metadata which is utilized for managing snapshots, tracking for remote replication, managing deduplication pointers, managing data compression, resiliency related metadata (e.g., RAID), etc. The metadata management system 240 manages the metadata generated by the storage control system 210.

The write cache management system 230 implements methods that are configured to provision and manage the write cache 262 in the primary memory 260 or in primary storage as provided by the array of storage devices 250. In some embodiments, the write cache 262 is utilized to serve I/O write requests by persistently storing data items (e.g., write data) together with associated metadata items in the write cache 262. In this regard, the write cache 262 reduces the overhead for handling write data and associated metadata since the write data and associated metadata is initially persisted in the write cache 262 without the need for extra I/O to store the metadata items separately from the data items. In addition, when the storage control system 210 receives an I/O write request and associated write data from a given host system, the storage control system 210 will send an acknowledgment to the host system to indicate that the write data is successfully written to the primary storage, in response to the received write data and associated metadata being stored in the write cache 262.

In some embodiments, the write cache 262 is implemented as a cyclic buffer, wherein items (e.g., data items and metadata items) are always written to a head location of the write cache 262, and items are destaged from a tail location of the write cache 262. With this scheme, the items in the write cache 262 are arranged in a cyclic write order from the tail location of the write cache 262 to the head location of the write cache 262. Further, in some embodiments, the write cache management system 230 utilizes a plurality of pointers in conjunction with the write cache 262 (e.g., cyclic write cache) to (i) determine the tail location and the head location of the write cache 262, (ii) determine a location in the write cache 262 from where to begin a recovery process, and to (iii) keep track of the data items and metadata items that are destaged from the tail of the write cache 262 via destage operations performed by the write cache destage control module 232.

More specifically, the write cache destage control module 232 implements write cache eviction/destaging operations which take into consideration that the write cache 262 comprises both data items and associated metadata items, which are separate entities that are persisted in different primary data structures. In some embodiments, the write cache destaging operations are configured to destage data items and destage metadata items, separately, based on associated eviction/destaging policies. For example, the metadata items destaged from the write cache 262 are persisted in the primary metadata structure 270 using techniques as discussed in further detail below.

The metadata management system 240 utilizes the write cache 262 in conjunction with the primary metadata structure 270 to persist, manage, and access metadata maintained by the storage node 200. Collectively, the write cache 262 and the primary metadata structure 270 provide a high-performance data structure that enables efficient management of metadata in the storage system. Various techniques for high-performance metadata management will now be discussed in further detail in conjunction with FIGS. 3-7 .

For example, FIG. 3 schematically illustrates a process for utilizing a high-performance data structure for managing metadata in a storage system, according to an exemplary embodiment of the disclosure. More specifically, FIG. 3 schematically illustrates a write cache 300, and a primary metadata structure 310 (e.g., storage metadata structure). The primary metadata structure 310 comprises an updates data structure 312, and a core data structure 314. As noted above, the write cache 300 is configured to initially persist data items (e.g., write data of I/O write requests) and metadata items to thereby serve I/O write requests with a minimal delay, and allow the storage control system 210 to return an acknowledgment to a host system when the write data of an I/O write request and associated metadata is successfully written to the write cache 300.

As shown in FIG. 3 , the write cache 300 comprises a plurality of data items 301, 303, 305, and 307, and a plurality of associated metadata items 302, 304, 306, and 308. In the illustrative embodiment of FIG. 3 , each metadata item 302, 304, 306, and 308 is labeled by a letter and a number. For instance, the metadata item 302 is labeled A5, the metadata item 304 is labeled F3, the metadata item 306 is labeled C2, and the metadata item 308 is labeled F4. The letter of a given metadata item represents a location of the data item (e.g., an offset in a file or volume, or an object ID) associated with the given metadata item, and the number represents a consecutive order of update of the data item (e.g., A5 denotes a next change to A4 for location “A”). In other words, the number represents a generation (or revision) number of the given data item. In an exemplary embodiment in which the allocation units (i.e., fixed-size addressable units) of the storage devices have a fixed “allocation unit size” of, e.g., 8 KB, and the letters represent offset locations in a given volume, then the following representations may apply: A→offset 0, B→offset 8 KB, C→offset 16 KB, D→offset 24 KB, E→offset 32 KB, etc.

Since writes to the write cache 300 are acknowledged to the host, the write cache 300 is configured to serve read requests for the cached data items that have not yet been destaged to the primary storage. To serve reads, the data in the write cache 300 must have lookup capabilities. However, the write cache 300 is optimized for writes and not for reads and, thus, the metadata in the write cache 300 is not random access. The lookup capability is provided by the updates data structure 312 and the core data structure 314. In other words, the primary metadata structure 310 provides a joint metadata structure to access data items in the write cache 300 before the data items are destaged and stored to primary storage (e.g., HDD/SSD primary storage devices of the storage system), as well as to access data items after they are destaged from the write cache 300 and stored to primary storage.

As noted above, in some embodiments, the write cache 300 is located on the same storage device as the data, while in other embodiments, the write cache 300 is allocated in a region of a persistent memory. In such instances, metadata is required to access both the primary storage and the write cache 300. The updates and core data structures as discussed herein are utilized to refer to storage and cache interchangeably. A data item that was originally placed in the write cache may be destaged, involving an update to a metadata item, using all the same structures. This is enabled by the ability to persist micro updates with negligible I/O overhead.

The updates data structure 312 is configured to accumulate changes to metadata and provide write amortization to the core data structure 314. As shown in FIG. 3 , the metadata items in the write cache 300 are added to the updates data structure 312 by performing a metadata destage operation 330 under control of the write cache destage control module 232. The metadata destage operation 330 involves writing copies of the metadata items in the write cache 300 to an in-memory buffer in RAM, which is allocated to the updates data structure management module 242 for temporarily buffering cached metadata items before persisting the metadata items in the updates data structure 312. When the in-memory buffer becomes full (e.g., the number or amount of metadata items in the buffer reaches a predetermined buffer size), the metadata items are persisted to the updates data structure 312. During the metadata destage operation 330, resiliency of the metadata items is provided by the write cache 300 until the buffered metadata items are persisted in the updates data structure 312. Once the metadata items are persisted in the updates data structure 312, the cached metadata items are no longer needed, and can be removed from the write cache 300.

As shown in FIG. 3 , the data and associated metadata items in the write cache 300 are arranged in a temporal order based on, e.g., an order in which user writes are received by the storage control system. In other words, assuming that the letters (e.g., A, F, C, F, etc.) of the data items and associated metadata items represent offset location in volume, the items in the write cache 300 are not arranged in “alphabetic order” but instead are arranged in temporal order. However, when the metadata items are destaged from the write cache 300 and persisted in the updates data structure 312, the metadata items are arranged in the updates data structure 312 in a sorted manner (e.g., alphabetical order) to facilitate indexing and searching of the metadata items.

To enable lookup using the updates data structure 312, the internal ordering of the updates data structure 312 is the same as the internal ordering of the core data structure 314 in that both data structures 312 and 314 utilize the same key (e.g., the keys (letters) which represent the offset positions in a given volume). In some embodiments, to support swapping of metadata, the updates data structure 312 comprises pages, each of which may or may not be in the system RAM. Each page comprises a unit of capacity that is allocated to store metadata items. For example, in some embodiments, the page size for the updates data structure 312 can be 512 bytes, 1,024 bytes (1 KB), 2,048 bytes (2 KB), 4,096 bytes (4 KB), 8,192 bytes (8 KB), etc.

The core data structure 314 is a metadata storage and indexing structure, which is configured to contain all the metadata except the most recent metadata that is contained in the updates data structure 312. The core data structure 314 is essentially a semi-static data structure since all metadata updates are first accumulated in the updates data structure 312, and then eventually persisted to the core data structure 314 using a bulk update operation 340, as schematically shown in FIG. 3 . The updates data structure 312 essentially serves as a journal which persists and accumulates most recent changes of the metadata items, and then over time, the metadata is sorted into the core data structure 314. The bulk update operation 340 is occasionally performed to apply metadata updates from the updates data structure 312 to the core data structure 314. The occasional bulk update operation 340 leads to large-scale changes to the core data structure 314, which allows for many optimizations including, but not limited to, write amortization, tree balancing, removal of deleted items, constructing new inner nodes of the core data structure 314 when write-in-place is avoided.

In some embodiments, the updates data structure 312 is implemented using an LSM tree data structure, and the core data structure 314 is implemented using a B+ tree data structure. The combined use of the LSM tree and B+ tree data structure provides a high-performance data structure for managing metadata. More specifically, the B+ tree data structure is utilized to implement a primary lookup and address mapping functionality, while the LSM tree data structure is configured to provide write amortization for metadata updates (e.g., adding new metadata items, modifying metadata items, deleting/invalidating metadata items. The LSM tree data structure allows for low-latency writing of metadata to a persistent data structure using out-of-place updates in which metadata items (new metadata items, updated metadata items) are always stored to new locations instead of overwriting old entries (eliminating the need for random I/Os for writes). The LSM tree data structure enhances write performance for persisting the metadata since the LSM tree data structure can exploit sequential I/Os to handle a high rate of write operations and, thereby, allow metadata to be quickly persisted to support high-throughput writes. In addition, the LSM tree data structure comprises a multilevel tree-like structure which is configured to progressively store key-value pairs associated with metadata items to enable read access to the metadata items using binary searches, indexing structures, Bloom filters, etc.

The B+ tree data structure implements a metadata storage and indexing structure, which is configured to contain all the metadata except the most recent metadata updates that are accumulated in the LSM data structure. The B+ tree data structure is optimized for read access to metadata items as the B+ tree data structure allows random or sequential I/O access to the metadata items. However, a B+ tree data structure is not optimized for write operations, as the B+ tree data structure is typically configured for in-place updates in which old records are overwritten to store new updates. In this regard, the use of a B+ tree data structure alone to manage metadata would sacrifice write performance, as updates to the B+ tree data structure incur random I/Os as every write to the B+ tree data structure has to perform multiple write operations including a write of the given record, as well as performing required metadata updates to the B+ tree data structure with regard to moving, splitting and merging nodes in the B+ tree data structure, whereby resulting in slower writes. In this regard, the LSM tree data structure is utilized to accumulate metadata changes with high write performance, and heavily buffer such metadata changes for subsequent addition to the B+ tree data structure using bulk update operations outside the I/O processing path. Exemplary embodiments of LSM and B+ tree data structures will be discussed in further detail below in conjunction with FIGS. 4, 5, and 6 .

FIG. 4 schematically illustrates an exemplary LSM tree data structure 400 which is utilized to implement a high-performance data structure for managing metadata in a storage system, according to an exemplary embodiment of the disclosure. The exemplary LSM tree data structure 400 implements an in-memory buffer 402, and comprises Bloom filters 404, and a multilevel structure comprising a plurality of segment levels, e.g., Level 0, Level 1, and Level 2. The LSM tree data structure 400 is configured to enable write amortization for accumulating changes to metadata items (e.g., adding new metadata items, updating metadata items, and deleting metadata items) which are first written to the persistent write cache.

The in-memory buffer 402 comprises an in-memory structure that buffers metadata items that are destaged from the write cache. As noted above, metadata items in the write cache are destaged by copying the metadata items to the in-memory buffer 402. In some embodiments, the metadata items are inserted into the in-memory buffer 402 and sorted in order (e.g., by offset location, and generation number). When the in-memory buffer 402 reaches a predefined threshold size (e.g., allocation unit size, page size, etc.), the accumulated metadata items within the in-memory buffer 402 are flushed to a Level 0 segment of the LSM tree data structure 400.

For example, as shown in FIG. 4 , the LSM tree data structure 400 comprises a plurality of Level 0 segments 410 (or root segments) comprising a first segment 411, a second segment 412, a third segment 413, etc. Each Level 0 segment 410 comprises a persistent immutable data structure which stores a plurality of metadata items that are flushed from the in-memory buffer 402 at a given time. The process of updating the LSM tree data structure 400 prevents a high rate of updates to the Level 0 layer of the LSM tree data structure 400 by accumulating metadata changes (e.g., hundreds) in the write cache before the metadata items are written persistently from the in-memory buffer 402 to new Level 0 segment at once. Once the metadata items are persisted in a Level 0 segment 410, the metadata items in the write cache can be deleted.

More specifically, in the exemplary embodiment of FIG. 4 , the first segment 411 comprises sorted metadata items (A5, C2, F3, F4), the second segment 412 comprises sorted metadata items (A6, B4, E5, G2), and the third segment 413 comprises sorted metadata items (E6, H2, J5, K3). While FIG. 4 shows each Level 0 segment 410, 411, and 412 having four (4) metadata items for ease of illustration, each Level 0 segment 410 can have hundreds of metadata items (e.g., the Level 0 segments 411, 412, and 413 can each have a size of 8 KB (similar to the allocation unit size, or page size, etc.). The first segment 411 comprises metadata items F3 and F4 where F4 denotes a recent change to data at offset location F from the previous version F3. In addition, the first and second segments 411 and 412 include respective metadata items A5 and A6, where A6 denotes a recent change to data at offset location A from the previous version A5. Further, the second and third segments 412 and 413 include respective metadata items E5 and E6, where E6 denotes a recent change to data at offset location E from the previous version E5.

In this regard, the updated metadata items are written out-of-place by appending an update to the same or a new Level 0 segment as cached metadata items in the write cache are added to the LSM tree data structure 400. The older values for metadata items are removed at a later point in time through merge operations (or compaction operations). In the event of a crash while metadata items are still in the in-memory buffer 402, the write cache will be used to recover on the next restart.

The Level 0 segments 410 are continuously generated as more blocks of cached metadata items are flushed from the in-memory buffer 402. Periodically, multiple segments in a given Level (i) of the LSM tree data structure 400 are merged together into a single segment that is added to the next Level (i+1). The merging is performed to, e.g., reduce the number of segments that need to be searched. For example, as shown in FIG. 4 , the LSM tree data structure 400 comprises a plurality of Level 1 segments 420 including a first segment 421, a second segment 422, a third segment 423, etc., and a plurality of Level 2 segments 430 comprising a first segment 431, etc.

In the exemplary embodiment shown in FIG. 4 , the segments 411, 412, and 413 of the Level 0 segments 410 are merged together into the first segment 421 in Level 1 of the LSM tree data structure 400. The first segment 421 in Level 1 aggregates the metadata items from each of the Level 0 segments 411, 412, and 413, and stores the metadata items in a sorted order. Following the merge operation, the Level 0 segments 411, 412, and 413 (source segments) are removed from the LSM tree data structure 400. While not specifically shown in FIG. 4 , the second and third segments 422 and 423 in Level 1 of the LSM tree data structure 400 could each have a plurality of sorted entries aggregated from merging different blocks of three segments in Level 0 of the LSM tree data structure 400. Moreover, the first segment 431 in Level 2 of the LSM tree data structure 400 could have a plurality of sorted records as a result of merging the segments 421, 422, and 423 in Level 2 of the LSM tree data structure 400. The LSM tree data structure 400 uses very few writes to propagate entries from one level to the next by merging segments.

In some embodiments, each segment in the LSM tree data structure 400 is stored as a file, wherein the metadata items in a given segment are divided into fixed size pages (e.g., 8 KB blocks) of the file. More specifically, in some embodiments, each segment in each level of the LSM tree data structure 400 is divided into one or more pages which facilitates access, memory allocation, persistency, and swapping segments or portions of segments in and out of memory. For example, assume that each Level 0 segment 410 has a size equivalent to a page size (e.g., 8 KB), the segments of higher levels (e.g., Level 1, Level 2, etc.) will be increasingly larger in size and have multiple pages.

The pages are used for purposes of memory allocation and to facilitate lookup for a target metadata item in the LSM tree data structure 400. For example, each segment can have additional metadata with regard to the page(s) of the segment, wherein such metadata comprises a page key for each page in the segment, and an associated value which represents the key (e.g., offset location) of the first metadata item within the given segment. To locate a given entry in a given segment, the segment may include a page index which records a key of the first entry in each page. The page index together with the Bloom filters 404 provides for fast searching of metadata items in the LSM tree data structure 400.

In particular, when searching for a given metadata item which may be located in a given segment of the LSM tree data structure 400, the Bloom filters 404 will be used initially to enable efficient per-segment lookup. A Bloom filter for a given segment provides an indication that (i) the given segment may contain a key that is being searched for with a high probability, or that (ii) the given segment does not include the key that is being searched. In some embodiments, if a given segment is identified (via Bloom filtering) to likely have the target key, a binary search can be conducted to find and access the key within the given segment (if the key does in fact exist in the given segment). In some embodiments, for large sized segments, once a given segment is identified (via Bloom filtering) to most likely contain the target key, the page index for the given segment can be used to identify the page within the given segment which would have the key (if the key was indeed in the segment). Once the target page in the given segment is identified via the page index, the page can be loaded to memory and to perform a binary search of the keys in the target page to find the target key. In this regard, the first key of each page is maintained in RAM to reduce the search within a segment to a single page utilizing the fact that the segments are sorted.

FIG. 5 schematically illustrates an exemplary B+ tree data structure which is utilized to implement a high-performance data structure for managing metadata in a storage system, according to an exemplary embodiment of the disclosure. In particular, FIG. 5 schematically illustrates an exemplary B+ tree data structure 500 comprising three levels of nodes including a root level comprising a root node 501, an internal node level 510, and a leaf node level 520. The internal node level 510 comprises two internal nodes 511 and 512, and the leaf node level 520 comprises five leaf nodes 521, 522, 523, 524, and 525, wherein all leaf nodes are in the same level of the B+ tree data structure 500. For ease of illustration, FIG. 5 depicts an exemplary B+ tree data structure 500 which is configured, at a given point in time, to enable random access to stored data items in fourteen (14) offset locations identified by metadata keys A, B, C, D, E, F, G, H, I, J, K, L, M and N.

Furthermore, for ease of illustration, the exemplary B+ tree data structure 500 is an n-order tree structure, wherein n=3. The “order” of a B+ tree data structure denotes a maximum number of key fields that a given node can have. For n=3, the internal nodes and leaf nodes can have at most n=3 key fields and n+1=4 pointer fields. In addition, when n=3, each leaf node must have a least two (2) entries, and each non-leaf node must have at least one (1) entry and two (2) pointers. In practice, the order n of the B+ tree data structure 500 would be in the hundreds, or thousands, etc. In this regard, while the exemplary embodiment of FIG. 5 shows the exemplary B+ tree data structure 500 having three levels of nodes (the root node 501 level, the internal node level 510, and the leaf node level 520 level), it is to be understood that at any given time, there can be zero (0) to N internal levels of nodes. For example, as is known in the art, a B+ tree data structure grows from the leaf nodes upwardly. In this regard, at some period of time, the B+ tree may only have leaf nodes, with zero (0) non-leaf nodes.

In the exemplary B+ tree data structure 500, each non-leaf node 501, 511, and 512 includes one or more keys (e.g., offset locations), and each leaf node 521, 522, 523, 524, and 525 includes one or more key:value pairs. Each key:value pair comprises a key (represented by a letter) whose value represents, e.g., a logical offset location of the data in volume, and a value “loc” which identifies (e.g., pointer) a physical location of the data in the physical storage space. The B+ tree data structure 500 is configured to organize the metadata items in a manner which enables efficient mapping and lookup functionality for metadata items and associated data blocks as identified in the leaf nodes 521, 522, 523, 524, and 525. In the exemplary embodiment of FIG. 5 , the root node 501 comprises a key value J, with a pointer P1 to the internal node 511, and a pointer P2 to the internal node 512. The root node 501 comprises two empty key fields and two empty pointer fields, which can be used when new leaf nodes are added to the B+ tree data structure 500, resulting in a change in the structure of the inner nodes (root and internal nodes). The pointer P1 points to a branch of the B+ tree data structure 500 having nodes with key values that are less than the key value of J. The pointer P2 points to a branch of the B+ tree data structure 500 having nodes with key values that are greater than, or equal to, the key value J. More specifically, the pointer P1 points to the internal node 511, while the pointer P2 points to the internal node 512.

As further shown in FIG. 5 , the internal node 511 comprises key values D and G, and pointers P3, P4, and P5. The internal node 511 includes one empty key field and one empty pointer field, which can be used when new leaf nodes are added to the B+ tree data structure 500. The pointer P3 points to the leaf node 521 having a plurality of key:value pairs (e.g., A:loc; B:loc; C:loc), with key values A, B and C which are less than the key value D. The pointer P4 points to the leaf node 522 having a plurality of key:value pairs (e.g., D:loc; E:loc; F:loc), with key values D, E, and F which are greater than or equal to the key value of D, but less than the key value of G. The pointer P5 points to the leaf node 523 having a plurality of key:value pairs (e.g., G:loc; H:loc; I:loc), with key values G, H, and I which are greater than or equal to the key value of G, but less than the key value J.

Furthermore, the internal node 512 comprises a key value M and pointers P6 and P7. The internal node 512 comprises two empty key fields and two empty pointer fields, which can be used when new leaf nodes are added to the B+ tree data structure 500. The pointer P6 points to the leaf node 524 having a plurality of key:value pairs (e.g., J:loc; K:loc; L:loc), with key values J, K, and L which are less than the key value M. The pointer P7 points to the leaf node 525 having a plurality of key:value pairs (e.g., M:loc; N:loc) with key values M and N which are greater than or equal to the key value M. The leaf node 525 comprises an empty data record, which can be used when new leaf nodes are added to the B+ tree data structure 500. The B+ tree data structure 500 comprises a dynamic multilevel indexing data structure which can grow and shrink as leaf nodes are added or removed from the B+ tree data structure. The B+ tree data structure grows from the bottom up by the addition of new leaf nodes when data is written to storage using B+ tree insertion methods well-known to those of ordinary skill in the art.

In some embodiments, to support swapping, the nodes of the B+ tree data structure 500 (core data structure) are divided into pages similar to the LSM tree data structure 400 (updates data structure) as discussed above. In particular, the inner nodes and the leaf nodes of the B+ tree data structure 500 can be constructed from swappable pages. Even at a granularity of 4 KB, a reasonable system should be able to contain all the inner nodes of the B+ tree data structure in RAM, requiring only a single read to retrieve required metadata.

The updates data structure (e.g., LSM tree data structure 400) is optimized for writes, and handles individual writes. The LSM tree data structure can have multiple values per key. Insertions into the LSM tree data structure are lightly buffered by the write cache. On the other hand, the core data structure (e.g., B+ tree data structure 500) is optimized for reads, and is configured to handle bulk writes of updated metadata. The B+ tree data structure may have a single value per key. Insertions into the B+ tree data structure are heavily buffered by the updates data structure.

In some embodiments, the size of the updates data structure (e.g., LSM tree data structure 400) is determined by a compromise between two competing factors: (i) read performance and (ii) write performance. In particular, read performance is the highest when there is no entry in the updates data structure. In this regard, to enhance the overall read performance of the primary metadata structure, it is preferable to keep the updates data structure as small as possible. On the other hand, the overall write performance is greatest with a large updates data structure, so that the number of pages to update in the core data structure is small in relation to the number of changes being applied to the core data structure from the updates data structure. In some embodiments, a balance between read and write performance is achieved by keeping the size of the updates data structure to be approximately 1% the size of the core data structure. However, the size difference will depend on several factors such as the page size and the metadata entry size.

The exemplary embodiments discussed above assume that the granularity of the metadata of the updates data structure (e.g., LSM tree data structure) and core data structure (e.g., B+ tree data structure) is the same, e.g., 4 KB, 8 KB, etc. In other embodiments, the metadata granularity of the primary metadata structure is optimized for use in, e.g., logical-to-physical address mapping, wherein the metadata granularity of the update data structure is different from the metadata granularity of the core data structure.

The term “chunk” as used herein denotes a unit of data (data block) with a predefined size, e.g., an allocation unit size (chunk size) of the data storage system, wherein the predefined chunk size can be, e.g., 4 KB, 8 KB, 16 KB, etc. A chunk entity is utilized to manage data in the data storage system, wherein each data chunk is associated with (and referenced by) by a single metadata item which contains information for the data chunk. For example, the data of a given chunk can be (i) compressed as a single block (or item) of data, (ii) deduped as a single block of data, and (iii) stored as a single block of data. Further, the term “sub-chunk” as used herein refers to a block of data having a size which is less than the predefined (allocated) chunk size of the data storage system. For example, assuming the allocated chunk size is 8 KB, a block of data having a size of less than 8 KB is considered a sub-chunk.

The term “metadata granularity” with respect to metadata items generally denotes a size of a data item that is referenced by a single metadata item. For example, metadata items with a “chunk granularity” or “full chunk granularity” are metadata items that reference single data items with a full chunk size. In addition, metadata items with a “sub-chunk granularity” are metadata items that reference single data items with a size that is less than the predefined (allocated) chunk size of the data storage system. The terms “metadata item” and “metadata entry” as used herein are synonymous terms that can be used interchangeably.

While a block storage system may support a basic allocation unit size of 512 bytes (B), in certain applications, it is undesirable for the block storage system to manage metadata at a granularity of 512 B. For example, it may be more performance-efficient to configure the block storage system manage metadata at a higher granularity, e.g., 4 KB, 8 KB, 16 KB, etc. Even with the storage system configured at a higher granularity of, e.g., 8 KB, the storage system can still perform an I/O write operation for a sub-chunk of data (e.g., 512 bytes), which is a portion of an 8 KB block of data, using a read-modify-write operation. With the read-modify-write operation, an 8 KB block of data is read, and modified based on the I/O write data, and the modified 8 KB block is written back to storage. Thereafter, the metadata associated with the 8 KB data block is updated to include information based on the modified 8 KB block. This allows I/O write operations to be performed with I/O write data sizes that are smaller than full chunk sizes, e.g., I/O write of 512 bytes, or a multiple of 512 bytes less than the allocation unit size.

In some embodiments, the persistent write cache (e.g., write cache 300, FIG. 3 ) is configured to enable I/O writes at 512 B granularity, so that data can be written to the persistent write cache with sub-chunk granularity as compared to the data storage system which is configured to allow I/O writes at a predefined chunk granularity (e.g., 8 KB). When data is destaged from the persistent write cache and written to primary storage, the data will be written to the primary storage at full chunk granularity using, e.g., read-modify-write operations, even though the persistent write cache allows I/O writes at sub-chunk granularity. For example, when data items with sub-chunk sizes are destaged from the persistent write cache, such data items are written to 8 KB blocks in the primary storage.

In some embodiments, the primary metadata structure is configured to support metadata items having full chunk granularity and sub-chunk granularity. More specifically, in some embodiments, the updates data structure (e.g., LSM tree data structure) is configured to support a metadata granularity for sub-chunks of data, while the core data structure (e.g., B+ tree data structure) is configured to support a metadata granularity for full chunks of data. For example, the LSM tree data structure (updates data structure) is configured to store and manage metadata items that are associated with data items having sub-chunk or chunk sizes. In particular, as noted above, since data items with sub-chunk sizes can be written to the persistent write cache, the metadata items, which are associated with cached data items having sub-chunk sizes, are stored in the LSM tree data structure and point to the sub-chunk data items in the persistent write cache.

On the other hand, the B+ tree data structure (core data structure) is configured to store and manage metadata items that are associated with data items having the full chunk size. In this regard, the core data structure is configured to manage metadata items at full chunk granularity, which makes the core data structure more performant and more RAM efficient. With the exemplary primary data structure, the updates data structure is relatively much smaller in size than the core data structure (e.g., the size of the LSM tree data structure is about 1% the size of the B+ tree data structure). In this regard, configuring the LSM data structure to support a metadata granularity of 512 B is not performance prohibitive. Exemplary methods for managing metadata at sub-chunk and chunk granularities using a primary data structure comprising a joint LSM tree data structure and B+ tree data structure will now be discussed in further detail in conjunction with FIGS. 6 and 7 .

FIG. 6 schematically illustrates an exemplary LSM tree data structure 600 which is configured to manage metadata items at sub-chunk granularity, according to an exemplary embodiment of the disclosure. The LSM tree data structure 600 comprises a plurality of segment levels, e.g., Level 0, Level 1, and Level 2. In particular, a root level (Level 0) comprises a plurality of Level 0 segments 610 which include segments 611, 612, 613, 614, 615, 616, 617, 618, and 619. A next level (Level 1) comprises a plurality of Level 1 segments 620 which include segments 621, 622, 623, 624, 625, 626, 627, 628, and 629. A last level (Level 2) comprises a plurality of Level 2 segments 630 which include segments 631, 632, 633, 634, 635, 636, 637, 638, and 639. In addition, the Level 2 segments 630 include a sub-chunk segment 640, the functions of which will be explained in further detail below.

In the exemplary LSM tree data structure 600 shown in FIG. 6 , the shaded segments represent segments having entries of metadata items (wherein the items are sorted in order), while the unshaded segments represent unused segments (i.e., segments with no entries of metadata items due to merging of the entries to a next lower level). In FIG. 6 , the heights of the segments represent a relative size of the segments between the different levels. For example, based on the relative segment heights shown in FIG. 6 , the Level 1 segments 620 are greater in size than the Level 0 segments 610, and the Level 2 segments 630 are greater in size than the Level 1 segments 620. In addition, FIG. 6 shows that a size of the sub-chunk segment 640 is smaller than the size of the segments 631, 632, 633, 634, 635, 636, 637, 638, and 639 (e.g., the size of the sub-chunk segment 640 is the same or similar to the size of the Level 0 segments 610). In some embodiments, the Level 2 segments 630 can be very large (e.g., 100 MB or 10 GB), while the Level 0 segments 610 can be much smaller (e.g., 32 KB). While not specifically shown in FIG. 6 , each segment in the LSM tree data structure 600 comprises a respective Bloom filter to allow a fast lookup of entries of metadata items in the segments, using techniques as discussed above in conjunction with FIG. 4 .

In the LSM tree data structure 600, a group of segments in a given level i (which form a merge set) are merged into a single segment in a next level i+1 of the LSM tree data structure 600. For example, FIG. 6 shows dashed rectangular boxes that surround respective groups of segments (e.g., 3 segments), wherein each dashed rectangular box represents a “merge set.” More specifically, in the exemplary embodiment shown in FIG. 6 , in Level 0, the segments 611, 612, and 613 form a merge set M1, the segments 614, 615, and 616 form a merge set M2, and the segments 617, 618, and 619 form a merge set M3. Furthermore, in Level 1, the segments 621, 622, and 623 form a merge set M4, the segments 624, 625, and 626 form a merge set M5, and the segments 627, 628, and 629 form a merge set M6. Similarly, in Level 2, the segments 631, 632, and 633 form a merge set M7, the segments 634, 635, and 636 form a merge set M8, and the segments 637, 638, and 639 form a merge set M9. It is to be understood that the number of segments that are merged together (i.e., number of segments in a merge set) is implementation specific, and that FIG. 6 illustrates an exemplary, non-limiting embodiment in which each merge set is shown to include 3 segments for illustrative purposes.

FIG. 6 schematically illustrates exemplary merge operations 650 and 652 in which the metadata entries in segments of a given merge set are merged into a single segment in a next deeper level of the LSM tree data structure 600. In particular, the merge operation 650 involves merging metadata entries in the segments 611, 612, and 612 of the merge set M1 in Level 0 into the segment 626 in Level 1. In addition, the merge operation 652 involves merging the metadata entries in the segments 624, 625, and 626 of the merge set M5 in Level 1 into the segment 638 in Level 1. Once the source segments of a given merge set are merged into a new LSM, the entries of the source segments are removed from the LSM tree data structure 600. As part of the merge operations 650 and 652, older versions of metadata entries will be deleted when they are merged with newer versions of metadata entries having the same key value. Furthermore, when segments are merged, a new Bloom filter is created for the new segment, wherein the size of the new Bloom filter is proportional to the size of the new segment.

Moreover, FIG. 6 schematically illustrates an exemplary merge operation 654 (Level 2 merge operation) in which metadata entries in the sub-chunk segment 640, and metadata entries in the segments 631, 632, and 633 of the merge set M7 in Level 2 of the LSM tree data structure 600 are logically merged and sorted based on key value, to provide a set of merged and sorted entries 660. As part of the merge operation 654, older versions of metadata entries will be deleted when they are merged with newer versions of metadata entries having the same key value. For example, an older sub-chunk metadata entry can be deleted when another newer sub-chunk metadata entry or newer full chunk metadata entry with key value that matches the key value of the older sub-chunk metadata entry.

At the completion of the merge operation 654, the set of merged and sorted entries 660 will have metadata entries with full chunk granularity, which can be inserted into the B+ tree data structure (core data structure) by performing a core update operation. Furthermore, at the completion of the merge operation 654, the set of merged and sorted entries 660 may have one or more remaining metadata entries with sub-chunk granularity, which cannot be inserted into the B+ tree data structure as part of the core update operation. Since these remaining metadata entries with sub-chunk granularity cannot be added to the core data structure, such remaining sub-chunk metadata entries will be added to the sub-chunk segment 640, in sorted order of key value, to maintain the metadata information that may be needed to access the associated sub-chunk data items in the write cache as part of a read operation.

When the next Level 2 merge operation is performed for a next merge set (e.g., merge set M8), the sub-chunk metadata entries currently present in the sub-chunk segment 640 will be merged and sorted with metadata entries in the segments of the next merge set, and the process is repeated. There can be a circumstance in which a given sub-chunk metadata entry, which was included in a previous Level 2 merge operation and then added back into the sub-chunk segment 640, will be deleted as a result of the next Level 2 merge operation (or subsequent Level 2 merge operation) when, for example, the merged set of metadata entries includes a full chunk metadata entry having key value which matches the key value of the given sub-chunk metadata entry. In this instance, the full chunk metadata entry corresponds to a full chunk data item which has overwritten the sub-chunk data item associated with the given sub-chunk metadata entry.

In some embodiments, when an I/O write request is received by the storage control system, the I/O write data and an associated metadata item will be initially written to the persistent write cache, and the associated metadata item is written to an in-memory buffer and then subsequently written to a Level 0 segment 610 of the LSM tree data structure 600. For example, FIG. 6 shows an exemplary write process in which new metadata items are written to the segment 616 in Level 0. When the metadata items for the I/O write data are written to the LSM tree data structure 600, the storage control system will send an acknowledge (ACK) message to the host or user which issued the I/O write request. In this instance, the ACK message confirms that the I/O write data is successfully written in the persistent write cache, and that the associated metadata is successfully written in the persistent write cache and in the primary metadata structure. With this write process, if a given I/O read request is subsequently received for the data, the associated metadata will be in the primary metadata structure and may be utilized to access the data.

Depending on the size of the I/O write data, the associated metadata items will be written to a Level 0 segment with a sub-chunk granularity or full chunk granularity. For example, since the persistent write cache supports writing data at a sub-chunk granularity (e.g., 512 B), the LSM tree data structure 600 is configured to serve as a buffer to support the insertion of metadata items into the Level 0 segments 610 at a sub-chunk granularity of, e.g., 512 B for small I/O writes with sub-chunk sizes of 512 B, or multiples thereof, which is less than the chuck size (e.g., 8 KB) of the storage system. On the other hand, for large I/O writes, e.g., 8 KB, or multiples thereof, the associated metadata will be written to a Level 0 segment 610 with full chunk granularity.

When metadata items are initially written to Level 0 segments 610 of the LSM tree data structure, the metadata items will point to associated data items in the persistent write cache. For example, a given metadata item with sub-chunk granularity in a given Level 0 segment can point to a given sub-chunk of data (e.g., 512 B data item) in the persistent write cache. Eventually, the data items which are initially stored in the persistent write cache will be destaged from the persistent write cache and stored in primary storage. The data items that are destaged from the write cache (e.g., full chunk data items, and sub-chunk data items) are written to block storage (primary storage) at chunk granularity (e.g., 8 KB chunks) using read-modify-write operations, which effectively overwrite sub-chunk data items.

As a result of a read-modify-write operation, a new metadata entry with full chunk granularity will be inserted into a Level 0 segment of the LSM tree data structure. The new metadata entry, which is associated with a newly written full chunk data item, may have a key value that matches a key value of a metadata entry with sub-chunk granularity that was previously inserted into a Level 0 segment of the LSM tree data structure. In this instance, the matching key values indicates that the sub-chunk data item associated with the sub-chunk metadata entry has been overwritten by a full chunk data item associated with the new full chunk metadata entry.

In addition, new out of place writes of modified data items in the persistent cache, and the insertion of the associated metadata entries (for such modified data items) into the Level 0 segments 610 of the LSM tree data structure, will result in the previous (older) metadata entries with the same key values to become obsolete/invalid. As noted above, as metadata entries in the segments are merged and propagate down the LSM tree data structure, older versions of metadata entries will be deleted when they are merged with newer versions of metadata entries having the same key value. In this regard, the segment merge operations that are performed for the LSM tree data structure can result in the removal of the metadata entries with sub-chunk granularity, before such metadata entries with sub-chunk granularity reach a last (deepest) level of the LSM tree data structure.

For example, in the exemplary embodiment of FIG. 6 , assume that a given metadata item (denoted A1) associated with a given sub-chunk data item (which is written to the persistent write cache) is inserted into the segment 611 in Level 0 of the LSM tree data structure 600. Assume further that the given sub-chunk data item is subsequently destaged from the write cache and written (at chunk granularity) to an 8 KB block the primary storage, and that an updated metadata item (denoted A2) associated with the given data item is inserted into the segment 613 in Level 0 of the LSM tree data structure 600. The merge operation 650 will aggregate the metadata items from each of the Level 0 segments 611, 612, and 613, and store the metadata items in a sorted order in the segment 626 in Level 1. In this instance, since the segments 611 and 613 are part of the same merge set M1, and since the metadata items A1 and A2 have the same key value, the newer metadata item A2 (at full chunk granularity) is deemed to have overwritten the older metadata item A1 (at sub-chunk granularity), and the older metadata item A1 will be deleted and not included in the merged set of sorted metadata items in the segment 626.

Despite the existence of updated metadata items with full chunk granularity in segments of the LSM tree data structure 600, it is possible for a given metadata item with sub-chunk granularity to reach the bottom (deepest) level, e.g., Level 2, of the LSM tree data structure before being deleted as a result of a merging operation. For example, assume that the updated metadata item A2 associated with the given data item is inserted into the segment 614 in Level 0 of the LSM tree data structure 600, which is outside the boundary of the first merge set M1. In this instance, the merge operation 650 will result in the metadata item A1 (with sub-chunk granularity) in the segment 611 being inserted into the segment 626 in Level 1, despite the existence of the updated metadata item A2 (with chunk granularity) in the segment 614 in the higher level, Level 0, of the LSM tree data structure 600. Furthermore, as a result of the merge operation 652, the metadata item A1 (with sub-chunk granularity) in the segment 626 will be added to the segment 638 in Level 2 of the LSM tree data structure 600.

Moreover, as a result of another merge operation (not specifically shown in FIG. 6 ), the metadata item A2 (with chunk granularity) in the segment 614 may be added to the segment 627 in Level 1 when the metadata items in the segments 614, 615, and 616 of the merge set M2 are merged into the segment 627 in Level 1. In addition, as a result of another merge operation (not specifically shown in FIG. 6 ), the metadata item A2 in the segment 627 in Level 1 may be added to the segment 639 in Level 2 when the metadata items in the segments 627, 628, and 629 of the merge set M6 are merged into the segment 639 in Level 2.

In a subsequent merge operation in which the metadata items in the segment 627, 638, and 639 of the merge set M9 are merged together for insertion into the core data structure, the merged set of metadata items will include the metadata items A1 and A2 with matching key values, which results in the deletion of the metadata item A1 (with sub-chunk granularity), while the metadata item A2 (with chunk granularity) can be inserted in the core data structure. On the other hand, if the metadata item A2 did not reach Level 2 or was otherwise not present in the merge set M9 at the time of a Level 2 merge operation in which the metadata items in the merge set M9 were being added to the core data structure, the metadata item A1 with sub-chunk granularity would be inserted into the sub-chunk segment 640.

It is to be noted that the sub-chunk segment 640 is essentially a Level 2 segment which is dedicated to storing metadata items with sub-chunk granularity, which are present in the Level 2 segments 630, and which cannot be added to the core data structure. In this instance, when a merge operation occurs in Level 2 to merge the metadata items in a given merge set, which are to be inserted into the core data structure (B+ tree data structure), the metadata items with sub-chunk granularity are “sideways” merged into the sub-chunk segment 640 in Level 2 of the LSM tree data structure 600, rather than added to the core data structure. Essentially, the sub-chunk segment 640 logically operates as a regular segment in the LSM tree data structure 600, except that the sub-chunk segment 640 is configured to store only metadata items with sub-chunk granularity, which cannot be added to the core data structure as part of a core update operation. As with other segments in Levels 0, 1, and 2 of the LSM tree data structure 600, the metadata items in the sub-chunk segment 640 are sorted according to key value.

Moreover, since the metadata items in the sub-chunk segment 640 always correspond to the oldest metadata entries in the LSM tree data structure 600, the sub-chunk segment 640 is positioned as the beginning segment of Level 2 (e.g., leftmost position as shown in FIG. 6 ). As noted above, for each merge operation that is performed at Level 2 (deepest level) of the LSM tree data structure 600, the sub-chunk metadata entries in the sub-chunk segment 640 are included in as part of the merge operation. For each sub-chunk metadata entry, the merge operation determines whether there is a newer metadata item (e.g., having chunk granularity) in the set of merged metadata items, which has a matching key value to the sub-chunk metadata entry, and will either delete the sub-chunk metadata entry (if a newer metadata item with a matching key is present in the merge set), or insert the sub-chunk metadata entry into the sub-chunk segment 640 (if a newer metadata item with a matching key is not present in the merge set).

It is to be noted that the sub-chunk segment 640 does not have to be the size of a Level 2 segment 630. For example, the Level 2 segments 630 can be very large (e.g., 100 MB or 10 GB). On the other hand, the sub-chunk segment 640 can be much smaller (e.g., 5 MB) than the size of the Level 2 segments 630, as the sub-chunk segment 640 is only utilized to buffer metadata items with sub-chunk granularity. Indeed, a smaller size sub-chunk segment 640 is sufficient since an updated metadata item, which is associated with a full chunk overwrite of a sub-chunk data item, is expected to be present in a Level 2 segment 630 during a next Level 2 merge operation (one of next several Level 2 merge operations). Therefore, even a small-sized sub-chunk segment 640 is sufficient to temporarily store metadata items at sub-chunk granularity for the relatively short time that it may take for corresponding updated metadata items at full chunk granularity to propagate down to the last level (e.g., Level 2) of the LSM tree data structure 600 and free the metadata entries for the sub-chunk data items from the sub-chunk segment 640.

FIG. 7 illustrates a flow diagram of a method for managing metadata using a metadata structure that is configured to manage metadata items at chunk and sub-chunk granularities, according to an exemplary embodiment of the disclosure. More specially, FIG. 7 is a flow diagram of a process for utilizing a metadata structure which comprises an LSM tree data structure, such as shown in FIG. 6 , for managing metadata items at sub-chunk granularity, and a B+ tree data structure for managing metadata items at full chunk granularity, according to an exemplary embodiment of the disclosure. For purposes of illustration, FIG. 7 will be discussed in the context of the exemplary LSM tree data structure 600 of FIG. 6 .

The process flow in FIG. 7 begins by performing a merge operation to aggregate (e.g., sort and compact) metadata entries of a plurality of segments in a given level of the LSM tree data structure 600 (block 700). The merge operation (in block 700) can be a merge operation that is performed in the last level (e.g., Level 2) of the LSM tree data structure, such as shown in FIG. 6 , where the metadata entries in the sub-chunk segment 640 and in the merge set M7 are logically merged and sorted according to key value. For the merge operation, older metadata entries with key values that match corresponding key values of newer metadata entries are deleted. In this regard, if the aggregated metadata entries initially include one or more metadata entries with sub-chunk granularity (sub-chunk metadata entries), the merge operation may result in the deletion of one or more sub-chunk metadata entries.

Next, if the merge operation was not performed in the last level (e.g., Level 2) of the LSM tree data structure 600 (negative determination in block 701), the aggregated metadata entries are inserted in a new segment in a next level of the LSM tree data structure (block 702), and the merge operation is complete. In this instance, since the aggregated metadata entries are inserted into a segment of the LSM tree data structure, the aggregated metadata entries can include metadata entries at fully chunk granularity and metadata entries at sub-chunk granularity.

On the other hand, if the merge operation was performed in the last level (e.g., Level 2) of the LSM tree data structure (affirmative determination in block 702), a determination is made as to whether the aggregated metadata entries include any remaining metadata entries with sub-chunk granularity (sub-chunk metadata entries), which have not been deleted/overwritten as a result of the merge process (block 703). If the aggregated metadata entries do not include any remaining sub-chunk metadata entries, i.e., the aggregated metadata entries include only metadata entries with full chunk granularity (negative determination in block 703), the aggregated metadata entries with full chunk granularity are inserted into the B+ tree data structure (block 704).

However, if the aggregated metadata entries do include one or more remaining sub-chunk metadata entries (affirmative determination in block 703), the one or more remaining sub-chunk metadata entries are merged and inserted into the sub-chunk segment 640 in order of key value (block 705). In this instance, the aggregated metadata entries will include only metadata entries with full chunk granularity. The aggregated metadata entries with full chunk granularity are then inserted into the B+ tree data structure (block 704).

FIG. 8 schematically illustrates a framework of a server node for hosting a storage node which comprises a metadata management system, according to an exemplary embodiment of the disclosure. The server node 800 comprises processors 802, storage interface circuitry 804, network interface circuitry 806, virtualization resources 808, system memory 810, and storage resources 816. The system memory 810 comprises volatile memory 812 and non-volatile memory 814. The processors 802 comprise one or more types of hardware processors that are configured to process program instructions and data to execute a native operating system (OS) and applications that run on the server node 800.

For example, the processors 802 may comprise one or more CPUs, microprocessors, microcontrollers, application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), and other types of processors, as well as portions or combinations of such processors. The term “processor” as used herein is intended to be broadly construed so as to include any type of processor that performs processing functions based on software, hardware, firmware, etc. For example, a “processor” is broadly construed so as to encompass all types of hardware processors including, for example, (i) general purpose processors which comprise “performance cores” (e.g., low latency cores), and (ii) workload-optimized processors, which comprise any possible combination of multiple “throughput cores” and/or multiple hardware-based accelerators. Examples of workload-optimized processors include, for example, graphics processing units (GPUs), digital signal processors (DSPs), system-on-chip (SoC), tensor processing units (TPUs), image processing units (IPUs), deep learning accelerators (DLAs), artificial intelligence (AI) accelerators, and other types of specialized processors or coprocessors that are configured to execute one or more fixed functions.

The storage interface circuitry 804 enables the processors 802 to interface and communicate with the system memory 810, the storage resources 816, and other local storage and off-infrastructure storage media, using one or more standard communication and/or storage control protocols to read data from or write data to volatile and non-volatile memory/storage devices. Such protocols include, but are not limited to, NVMe, PCIe, PATA, SATA, SAS, Fibre Channel, etc. The network interface circuitry 806 enables the server node 800 to interface and communicate with a network and other system components. The network interface circuitry 806 comprises network controllers such as network cards and resources (e.g., network interface controllers (NICs) (e.g., SmartNICs, RDMA-enabled NICs), Host Bus Adapter (HBA) cards, Host Channel Adapter (HCA) cards, I/O adaptors, converged Ethernet adaptors, etc.) to support communication protocols and interfaces including, but not limited to, PCIe, DMA and RDMA data transfer protocols, etc.

The virtualization resources 808 can be instantiated to execute one or more services or functions which are hosted by the server node 800. For example, the virtualization resources 808 can be configured to implement the various modules and functionalities of a storage control system and a metadata management system as discussed herein. In some embodiments, the virtualization resources 808 comprise virtual machines that are implemented using a hypervisor platform which executes on the server node 800, wherein one or more virtual machines can be instantiated to execute functions of the server node 800. As is known in the art, virtual machines are logical processing elements that may be instantiated on one or more physical processing elements (e.g., servers, computers, or other processing devices). That is, a “virtual machine” generally refers to a software implementation of a machine (i.e., a computer) that executes programs in a manner similar to that of a physical machine. Thus, different virtual machines can run different operating systems and multiple applications on the same physical computer.

A hypervisor is an example of what is more generally referred to as “virtualization infrastructure.” The hypervisor runs on physical infrastructure, e.g., CPUs and/or storage devices, of the server node 800, and emulates the CPUs, memory, hard disk, network and other hardware resources of the host system, enabling multiple virtual machines to share the resources. The hypervisor can emulate multiple virtual hardware platforms that are isolated from each other, allowing virtual machines to run, e.g., Linux and Windows Server operating systems on the same underlying physical host system. The underlying physical infrastructure may comprise one or more commercially available distributed processing platforms which are suitable for the target application.

In another embodiment, the virtualization resources 808 comprise containers such as Docker containers or other types of Linux containers (LXCs). As is known in the art, in a container-based application framework, each application container comprises a separate application and associated dependencies and other components to provide a complete filesystem, but shares the kernel functions of a host operating system with the other application containers. Each application container executes as an isolated process in user space of a host operating system. In particular, a container system utilizes an underlying operating system that provides the basic services to all containerized applications using virtual-memory support for isolation. One or more containers can be instantiated to execute one or more applications or functions of the server node 800 as well as to execute one or more of the various modules and functionalities of a storage control system as discussed herein. In yet another embodiment, containers may be used in combination with other virtualization infrastructure such as virtual machines implemented using a hypervisor, wherein Docker containers or other types of LXCs are configured to run on virtual machines in a multi-tenant environment.

In some embodiments, the constituent components and modules of the storage nodes and storage control systems as discussed herein, as well as the metadata management methods as discussed herein, are implemented using program code that is loaded into the system memory 810 (e.g., volatile memory 812), and executed by the processors 802 to perform respective functions as described herein. In this regard, the system memory 810, the storage resources 816, and other memory or storage resources as described herein, which have program code and data tangibly embodied thereon, are examples of what is more generally referred to herein as “processor-readable storage media” that store executable program code of one or more software programs. Articles of manufacture comprising such processor-readable storage media are considered embodiments of the disclosure. An article of manufacture may comprise, for example, a storage device such as a storage disk, a storage array or an integrated circuit containing memory. The term “article of manufacture” as used herein should be understood to exclude transitory, propagating signals.

The system memory 810 comprises various types of memory such as volatile RAM, NVRAM, or other types of memory, in any combination. The volatile memory 812 may be a dynamic random-access memory (DRAM) (e.g., DRAM DIMM (Dual In-line Memory Module), or other forms of volatile RAM. The non-volatile memory 814 may comprise one or more of NAND Flash storage devices, SSD devices, or other types of next generation non-volatile memory (NGNVM) devices. The system memory 810 can be implemented using a hierarchical memory tier structure wherein the volatile memory 812 is configured as the highest-level memory tier, and the non-volatile memory 814 (and other additional non-volatile memory devices which comprise storage-class memory) is configured as a lower level memory tier which is utilized as a high-speed load/store non-volatile memory device on a processor memory bus (i.e., data is accessed with loads and stores, instead of with I/O reads and writes). The term “memory” or “system memory” as used herein refers to volatile and/or non-volatile memory which is utilized to store application program instructions that are read and processed by the processors 802 to execute a native operating system and one or more applications or processes hosted by the server node 800, and to temporarily store data that is utilized and/or generated by the native OS and application programs and processes running on the server node 800. The storage resources 816 can include one or more HDDs, SSD storage devices, etc.

It is to be understood that the above-described embodiments of the disclosure are presented for purposes of illustration only. Many variations may be made in the particular arrangements shown. For example, although described in the context of particular system and device configurations, the techniques are applicable to a wide variety of other types of information processing systems, computing systems, data storage systems, processing devices and distributed virtual infrastructure arrangements. In addition, any simplifying assumptions made above in the course of describing the illustrative embodiments should also be viewed as exemplary rather than as requirements or limitations of such embodiments. Numerous other alternative embodiments within the scope of the appended claims will be readily apparent to those skilled in the art. 

What is claimed is:
 1. A method, comprising: managing, by a storage control system, a storage metadata structure which comprises metadata items for accessing data items in a storage system comprising a persistent write cache and a primary storage, wherein the storage metadata structure comprises a first metadata structure that is configured to organize metadata items received from the persistent write cache at a first granularity, and a second metadata structure that is configured to organize metadata items received from the first metadata structure at a second granularity, wherein the second granularity is greater than the first granularity; and utilizing, by the storage control system, the storage metadata structure to access data items in one of the persistent write cache and the primary storage.
 2. The method of claim 1, wherein: the first granularity corresponds to a first allocation unit size of the persistent write cache, wherein each metadata item with the first granularity references a data item having the first allocation unit size; and the second granularity corresponds to a second allocation unit size of the primary storage, which is greater than the first allocation unit size of the persistent write cache, wherein each metadata item with the second granularity references a data item with the second allocation unit size.
 3. The method of claim 1, wherein the first metadata structure comprises a log-structured merge tree data structure, and the second metadata structure comprises a B+ tree data structure.
 4. The method of claim 1, wherein: the first metadata structure comprises a log-structured merge tree data structure comprising a plurality of levels; each level of the log-structured merge tree data structure comprises a plurality of segments which are configured to store metadata items in a sorted order; and a last level of the log-structured merge tree data structure comprises a designated sub-chunk segment which configured to receive and store metadata items at the first granularity, which are included in segments of the last level of the log-structured merge tree data structure, and which cannot be merged into the second metadata structure.
 5. The method of claim 4, wherein managing the storage metadata structure comprises: writing, by the storage control system, a first metadata item with the first granularity into a first level of the log-structured merge tree data structure, wherein the first metadata item references a first data item in the persistent write cache; writing, by the storage control system, the first data item in the persistent write cache into a second data item in the primary storage, wherein the second data item overwrites the first data item; and writing, the storage control system, a second metadata item with the second granularity into the first level of the log-structured merge tree, wherein the second metadata item references the second data item which overwrites the first data item.
 6. The method of claim 5, wherein managing the storage metadata structure further comprises: performing, by the storage control system, a merge operation to aggregate metadata items of each segment of a group of segments in the last level of the log-structured merge tree data structure, for insertion of the aggregated metadata items into the second metadata structure, wherein a given segment of the group of segments comprises the first metadata item; determining whether the second metadata item is included in a segment of the group of segments; and one of (i) inserting, by the storage control system, the first metadata item into the designated sub-chunk segment, in response to determining that the second metadata item is not included in a segment of the group of segments, and (ii) deleting, by the storage control system, the first metadata item, in response to determining that the second metadata item, which overwrites the first metadata item, is included in a segment of the group of segments.
 7. The method of claim 6, wherein managing the storage metadata structure further comprises: determining, by the storage control system, whether a third metadata item with the first granularity, which is present in the designated sub-chunk segment, has a key value that matches a key value of a newer metadata item in the group of segments; and deleting, by the storage control system, the third metadata item from the designated sub-chunk segment, in response to determining that the newer metadata item with the matching key value is present in the group of segments.
 8. The method of claim 1, further comprising: receiving, by the storage control system, a read request for a target data item; searching, by the storage control system, the storage metadata structure to find a target metadata item which references the target data item; and utilizing, by the storage control system, the target metadata item to access the target data item from one of the persistent write cache and the primary storage.
 9. An article of manufacture comprising a non-transitory processor-readable storage medium having stored therein program code of one or more software programs, wherein the program code is executable by one or more processors to implement a method which comprises: managing, by a storage control system, a storage metadata structure which comprises metadata items for accessing data items in a storage system comprising a persistent write cache and primary storage, wherein the storage metadata structure comprises a first metadata structure that is configured to organize metadata items received from the persistent write cache at a first granularity, and a second metadata structure that is configured to organize metadata items received from the first metadata structure at a second granularity, wherein the second granularity is greater than the first granularity; and utilizing, by the storage control system, the storage metadata structure to access data items in one of the persistent write cache and the primary storage.
 10. The article of manufacture of claim 9, wherein: the first granularity corresponds to a first allocation unit size of the persistent write cache, wherein each metadata item with the first granularity references a data item having the first allocation unit size; and the second granularity corresponds to a second allocation unit size of the primary storage, which is greater than the first allocation unit size of the persistent write cache, wherein each metadata item with the second granularity references a data item with the second allocation unit size.
 11. The article of manufacture of claim 9, wherein the first metadata structure comprises a log-structured merge tree data structure, and the second metadata structure comprises a B+ tree data structure.
 12. The article of manufacture of claim 9, wherein: the first metadata structure comprises a log-structured merge tree data structure comprising a plurality of levels; each level of the log-structured merge tree data structure comprises a plurality of segments which are configured to store metadata items in a sorted order; and a last level of the log-structured merge tree data structure comprises a designated sub-chunk segment which configured to receive and store metadata items at the first granularity, which are included in segments of the last level of the log-structured merge tree data structure, and which cannot be merged into the second metadata structure.
 13. The article of manufacture of claim 12, wherein the program code for managing the storage metadata structure comprises program code for: writing, by the storage control system, a first metadata item with the first granularity into a first level of the log-structured merge tree data structure, wherein the first metadata item references a first data item in the persistent write cache; writing, by the storage control system, the first data item in the persistent write cache into a second data item in the primary storage, wherein the second data item overwrites the first data item; and writing, the storage control system, a second metadata item with the second granularity into the first level of the log-structured merge tree, wherein the second metadata item references the second data item which overwrites the first data item.
 14. The article of manufacture of claim 13, wherein the program code for managing the storage metadata structure further comprises program code for: performing, by the storage control system, a merge operation to aggregate metadata items of each segment of a group of segments in the last level of the log-structured merge tree data structure, for insertion of the aggregated metadata items into the second metadata structure, wherein a given segment of the group of segments comprises the first metadata item; determining whether the second metadata item is included in a segment of the group of segments; and one of (i) inserting, by the storage control system, the first metadata item into the designated sub-chunk segment, in response to determining that the second metadata item is not included in a segment of the group of segments, and (ii) deleting, by the storage control system, the first metadata item, in response to determining that the second metadata item, which overwrites the first metadata item, is included in a segment of the group of segments.
 15. The article of manufacture of claim 14, wherein the program code for managing the storage metadata structure further comprises program code for: determining, by the storage control system, whether a third metadata item with the first granularity, which is present in the designated sub-chunk segment, has a key value that matches a key value of a newer metadata item in the group of segments; and deleting, by the storage control system, the third metadata item from the designated sub-chunk segment, in response to determining that the newer metadata item with the matching key value is present in the group of segments.
 16. An apparatus comprising: at least one processor; and memory configured to store program code, wherein the program code is executable by the at least one processor to instantiate a storage control system, wherein the storage control system is configured to: manage a storage metadata structure which comprises metadata items for accessing data items in a storage system comprising a persistent write cache and primary storage, wherein the storage metadata structure comprises a first metadata structure that is configured to organize metadata items received from the persistent write cache at a first granularity, and a second metadata structure that is configured to organize metadata items received from the first metadata structure at a second granularity, wherein the second granularity is greater than the first granularity; and utilize the storage metadata structure to access data items in one of the persistent write cache and the primary storage.
 17. The apparatus of claim 16, wherein: the first granularity corresponds to a first allocation unit size of the persistent write cache, wherein each metadata item with the first granularity references a data item having the first allocation unit size; and the second granularity corresponds to a second allocation unit size of the primary storage, which is greater than the first allocation unit size of the persistent write cache, wherein each metadata item with the second granularity references a data item with the second allocation unit size.
 18. The apparatus of claim 16, wherein: the first metadata structure comprises a log-structured merge tree data structure; the second metadata structure comprises a B+ tree data structure; the first metadata structure comprises a log-structured merge tree data structure comprising a plurality of levels; each level of the log-structured merge tree data structure comprises a plurality of segments which are configured to store metadata items in a sorted order; and a last level of the log-structured merge tree data structure comprises a designated sub-chunk segment which configured to receive and store metadata items at the first granularity, which are included in segments of the last level of the log-structured merge tree data structure, and which cannot be merged into the second metadata structure.
 19. The apparatus of claim 18, wherein in managing the storage metadata structure, the storage control system is configured to: write a first metadata item with the first granularity into a first level of the log-structured merge tree data structure, wherein the first metadata item references a first data item in the persistent write cache; write the first data item in the persistent write cache into a second data item in the primary storage, wherein the second data item overwrites the first data item; write a second metadata item with the second granularity into the first level of the log-structured merge tree, wherein the second metadata item references the second data item which overwrites the first data item; perform a merge operation to aggregate metadata items of each segment of a group of segments in the last level of the log-structured merge tree data structure, for insertion of the aggregated metadata items into the second metadata structure, wherein a given segment of the group of segments comprises the first metadata item; determine whether the second metadata item is included in a segment of the group of segments; and one of (i) insert the first metadata item into the designated sub-chunk segment, in response to determining that the second metadata item is not included in a segment of the group of segments, and (ii) delete the first metadata item, in response to determining that the second metadata item, which overwrites the first metadata item, is included in a segment of the group of segments.
 20. The apparatus of claim 19, wherein in managing storage the metadata structure, the storage control system is further configured to: determine whether a third metadata item with the first granularity, which is present in the designated sub-chunk segment, has a key value that matches a key value of a newer metadata item in the group of segments; and delete the third metadata item from the designated sub-chunk segment, in response to determining that the newer metadata item with the matching key value is present in the group of segments. 